To determine whether anti-IgE autoantibody (aIgE) is involved in modulating allergic asthma, we examined the relationship between the titers of aIgE and severity of asthma in 63 allergic asthmatics. The titer of aIgE in patients' sera was 14.4 +/- 17.9 units (mean +/- SD). Thirty-one patients had aIgE greater than 5 units (26.4 +/- 22.1 units), whereas 32 patients had aIgE less than 5 units (2.0 +/- 0.9 units). Patients with high aIgE (greater than 5 units) were less severe than those with low aIgE (less than 5 units) (1.65 +/- 0.42 and 2.19 +/- 0.52 of severity score, respectively; p less than 0.01). There was an inverse correlation between the titers of aIgE and the severity scores (r = -0.328; p less than 0.01). Patients with high aIgE also had lower bronchial reactivity to methacholine than those with low aIgE (556 +/- 292 and 170 +/- 182 micrograms/ml of threshold dose of inhaled methacholine, respectively; p less than 0.005). There was a significant correlation between the titers of aIgE and the threshold dose of methacholine (r = 0.499; p less than 0.01). These findings suggest that aIgE plays a protective role in bronchial hyperreactivity in allergic asthma.